This invention relates to new vulcanizable elastomeric compositions, and more particularly, to a method of providing a strong and durable bond between a vulcanizable elastomeric composition and a metal surface. The invention also relates to the laminates and rubber coated metal objects thus obtained.
In the production of rubber articles, such as pneumatic tires, belts, conveyor belts which contain reinforcing metallic elements, tubes provided with reinforcing cords or wires, and, in general, in the production of all rubber articles in which the rubber is reinforced with or bonded to a metal, it is necessary to obtain a strong and durable bond between the metal and the elastomeric composition in order to insure a long life for the articles produced.
In tires, for example, maximum reinforcement of the rubber is obtained when the maximum adhesion is produced between the laminate of rubber and the reinforcing element to form a unitary structure.
Considerable research has been conducted by those involved in the rubber industry toward achieving satisfactory rubber-to-metal bonding. The search for a strong and durable bond has continued over the years in view of the increasing demands placed on steel reinforced rubber as used in automobile and truck tires because of the use of such tires at higher speeds and higher loads. Various proposals have been made in the prior art of additives for rubbers which result in improved bonding between the rubbers and metal. For example, U.S. Pat. No. 2,720,479 describes a system wherein a phenolic resin and a brominated isoolefin-polyolefin interpolymer are dissolved in a suitable liquid carrier and the resulting adhesive composition is spread on the rubber which is to be bonded to the metal. The rubber and metal are subsequently pressed together and vulcanized.
U.S. Pat. No. 3,517,722 to Endter et al describes a rubber-metal adhesion system which involves formation of a resorcinol-formaldehyde resin at the interface between the rubber and the metal, thereby bonding these materials together. Upon vulcanization, methylene and resorcinol are released and presumably react to form the resorcinol-formaldehyde resin. In U.S. Pat. Nos. 3,256,137 and 3,266,970, resorcinol-aldehyde condensation products have been suggested along with certain methylene donors to promote adhesion of rubber to textiles.
More recently, it was suggested in U.S. Pat. No. 3,847,727, that the adhesion of rubber to metal such as wire cord is improved by incorporating a halogenated quinone (e.g., chloranil) and a resorcinol-aldehyde condensate into the rubber. Another resorcinol-formaldehyde resin based additive system is described in U.S. Pat. No. 3,862,883. The adhesive system of this patent utilizes a halogen-donating material such as a halogenated hydantoin in combination with the resorcinol-formaldehyde resin.
The use of organo-nickel salts as adhesion promotors in vulcanizable elastomers is suggested in U.S. Pat. No. 3,991,130. The nickel is present in a free valent state or in a metal complex associated with an organic ligand and certain specified anions.
The use of other metal organic salts such as the metal salts of organo carboxylic acids have been suggested in several patents. For example, U.S. Pat. No. 3,897,583 suggests that the adhesion of rubber to metal is improved by incorporating a cobalt salt such as cobalt naphthenate into a rubber stock which contains an adhesive resin forming system comprising a methylene donor and a resorcinol type methylene acceptor. Japanese Pat. No. 49-17661, published May 2, 1974, describes the use of zirconium compounds such as zirconium oxide, zirconium carbonate, zirconium octylate, zirconium stearate and zirconium tall oil fatty acid salt for improving the adhesion of steel cord to rubber. In the tests reported in the Japanese patent, the zirconium compounds improved the adhesion of a vulcanizable rubber composition more than did cobalt naphthenate. Cobalt salts of organic carboxylic acids in combination with sulphur have been suggested as improving rubber to metal adhesion in U.S. Pat. No. 3,514,370.
As mentioned above, because of the increasing demands placed on the metal reinforced rubbers such as automobile tires, there continues to be a need for improved adhesion of the rubber to metal. Moreover, some of the widely used additives in rubber for promoting the cure rate of the rubbers and improving the adhesion characteristics are the cobalt salts of organic carboxylic acids such as cobalt naphthenate which are quite expensive. Accordingly, there continues to be a need for new and inexpensive rubber additives.